1. Field of the Invention
This invention relates to methods for fabricating semiconductor integrated circuit structures utilizing both diffusion as well as ion implantation. In particular, it relates to the formation of ion implanted resistor regions during the fabrication of other devices within the chip, such as transistros having diffused base regions.
2. Description of the Prior Art
The method of forming semiconductor devices utilizing ion implantation has received a great deal of attention in recent years as a potential substitute for standard diffusion processes. The primary advantage of ion implantation as compared to diffusion is said to be the greater control of the area of the active region to be formed within the semiconductor, as well as the doping level. Thus, while diffusion technology has been satisfactory for the formation of impurity regions within the semiconductor substrate, it is thought that ion implantation will be required for more advanced devices. However, diffusion technology is well established and continues to be used.
It has been demonstrated that ion implantation is better than diffusion in the formation of resistor regions within the substrate particularly resistors with high resistivity. Such high valued resistors require low concentration levels, and it is difficult to obtain this with diffusion. Controlling the resistance value of resistors using thermal diffusion is difficult, as the spread of values using selected diffusion parameters is often greater than can be accepted for modern semiconductor circuits. These problems are substantially lessened if resistors are made by ion implantation.
However, even with the use of ion implantation for forming all of the impurity regions within a semiconductor substrate, a thermal cycle, commonly termed annealing, is required. For example, the process for forming the emitter region of a transistor with ion implantation is best accomplished by performing what is termed a predeposition ion implantation step followed by an annealing cycle of at least 1000.degree. C. for one hour to rearrange the impurities within the emitter region. It has been recognized that this thermal cycle could cause problems with the resistor regions if they were formed prior to or simultaneously with the formation of the emitter. Thus, it has been the standard practice within the industry to form the resistor region after the formation of all other regions which require thermal cycling for their formation. However, this arrangement requires in general more processing steps due to the need for a greater number of masks. In addition, because the maximum concentration of the implanted ions of the resistor are not at the surface there are problems with regard to the stability of the resistors.
In the last few years, those skilled in the art have contemplated using the annealing or diffusion step of the emitter regions to also effect the annealing of previously implanted resistor regions. See, for example, U.S. Pat. No. 3,933,528 issued in the name of B. J. Sloan, Jr. However, these efforts have been confined to simultaneous or successive implantations of the various regions, e.g., the base and resistor regions. It would be desirable to utilize this type of technique in cases where the base or other regions are diffused, rather than ion implanted. In particular, it is desirable that such a process require a minimum number of masks to form the various impurity regions.